Hadees-e-Jibrael

Umar ibn Khattab (Allah be well pleased with him) said:
“As we were sitting one day before the Messenger of Allah (peace and blessings be upon him), a man suddenly appeared. He wore pure white clothes and his hair was dark black—yet there were no signs of travel on him, and none of us knew him.
He came and sat down in front of the Prophet (peace and blessings be upon him), placing his knees against his, and his hands on his thighs. He said, “O Muhammad! Tell me about Islam.”